Rapid growth has depleted the excess capacity of the Central Kitsap activated sludge treatment plant advancing the time for upgrading much earlier than had been anticipated. This prompted the County to evaluate opportunities of optimizing and/or re-rating plant operations to identify where best to deploy scarce resources. The following group of measurement procedures and modeling tools were applied in pursuit of this goal: (i) verification of data collected through an audit of the CKWWTP laboratory and a calibration check of the major flow meters; (ii) clarifier testing through flocculation and stress testing of the secondary clarifiers; (iii) hydraulic and biological process modeling; (iv) performance verification over extended periods to compare plant operation against that predicted by the simulator. Sludge settleability was identified as a major capacity concern since the theoretical capacity of the plant's two secondary clarifiers decreased from 38 to 23 to 14 MGD as the SVI increased from 100 to 200 to 300 mL/g, respectively. Operational modes were identified with the aid of a calibrated biological process simulator that would eliminate denitrification-induced loss of solids from the secondary clarifiers. Full-scale testing validated these predictions. The combination of the above procedures provided justification for re-rating the plant from a peak month flow of 6.0 MGD to 7.0 MGD.